1. Field of the Invention
The present disclosure relates to heat exchangers, and more particularly to tubes and manifolds such as used in shell and tube heat exchangers.
2. Description of Related Art
Traditional tube shell heat exchangers have been designed with manifolds with cylindrical cross-sections to handle high pressures. An example of such a manifold 10 is shown in FIG. 1. When these heat exchangers are subjected to rapid changes in temperature of the high pressure fluid, there are significant temperature gradients in the assembly with resultant high stresses and strains into the plastic region at the tube/manifold interface that can ultimately result in cracking of the heat exchanger, severely shortening the useful life of the unit.
There is often a higher heat transfer coefficient near the high pressure fluid tube inlets (the small tube openings within the larger cylindrical manifold 10 shown in FIG. 1) due to a vena contracta within each tube near the tube inlet. The heat transfer coefficient in each tube has a peak value near the vena contracta and reduces in magnitude within each tube downstream of the vena contracta. The vena contracta effect causes high heat transfer coefficients at the tube/manifold interface, making this interface a location of peak thermally induced stress and strain.
In one particular version of a tube shell heat exchanger, the multiple tubes exiting the high pressure cylindrical manifold are parallel to each other, with the tubes furthest from the manifold centerline being more tangent to the manifold inner diameter, such as the lower most tubes as oriented in FIG. 1. Many of the tubes are cut to leave a distance between the inner manifold surface and tube end (referred to as standoff), with the tube ends roughly parallel to the inner manifold surface. The result is the tubes closer to tangent to the manifold inner diameter having a sharper point, i.e., having ends cut an angle further from normal to the tube's flow axis compared to tubes near the centerline of the manifold. This in turn results in larger vena contracta effects in these tangent tubes, with resultant high velocities on the tube wall opposite the vena contracta, high heat transfer coefficients, high thermal gradients, and high plastic strains during thermal transients. The more tangent tubes, e.g., the tubes near the bottom of the device shown in FIG. 1, present a design limitation for heat exchangers of this type, since the greatest thermal stress and strain tend to occur at the tube/manifold interface for these tubes.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for improved tubes and manifolds for heat exchangers. The present disclosure provides a solution for this need.